Characterization of the interaction between CD45 and CD45-AP

Enforced Dimerization of CD45 by the Adenovirus E3/49K Protein Inhibits T Cell Receptor Signaling

Human adenoviruses (HAdVs) are widespread pathogens that generally cause mild infections in immunocompetent individuals but severe or even fatal diseases in immunocompromised patients. In order to counteract the host immune defenses, HAdVs encode various immunomodulatory proteins in the early transcription unit 3 (E3). The E3/49K protein is a highly glycosylated type I transmembrane protein uniquely expressed by species D HAdVs. Its N-terminal ectodomain sec49K is released by metalloprotease-mediated shedding at the cell surface and binds to the receptor-like protein tyrosine phosphatase CD45, a critical regulator of leukocyte activation and functions. It remained elusive which domains of CD45 and E3/49K are involved in the interaction and whether such an interaction can also occur on the cell surface with membrane-anchored full-length E3/49K. Here, we show that the two extracellular domains R1 and R2 of E3/49K bind to the same site in the domain d3 of CD45. This interaction enforces the dimerization of CD45, causing the inhibition of T cell receptor signaling. Intriguingly, the membrane-anchored E3/49K appears to be designed like a "molecular fishing rod" using an extended disordered region of E3/49K as a "fishing line" to bridge the distance between the plasma membrane of infected cells and the CD45 binding site on T cells to effectively position the domains R1 and R2 as baits for CD45 binding. This design strongly suggests that both secreted sec49K as well as membrane-anchored full-length E3/49K have immunomodulatory functions. The forced dimerization of CD45 may be applied as a therapeutic strategy in chronic inflammatory disorders and cancer. IMPORTANCE The battle between viruses and their hosts is an ongoing arms race. Whereas the host tries to detect and eliminate the virus, the latter counteracts such antiviral measures to replicate and spread. Adenoviruses have evolved various mechanisms to evade the human immune response. The E3/49K protein of species D adenoviruses mediates the inhibition of immune cell function via binding to the protein tyrosine phosphatase CD45. Here, we show that E3/49K triggers the dimerization of CD45 and thereby inhibits its phosphatase activity. Intriguingly, the membrane-anchored E3/49K seems to be designed like a "molecular fishing rod" with the two CD45 binding domains of E3/49K as baits positioned at the end of an extended disordered region reminiscent of a fishing line. The adenoviral strategy to inhibit CD45 activity by forced dimerization may be used for therapeutic intervention in autoimmune diseases or to prevent graft rejection after transplantation.

Single-particle analysis of tear fluid reveals abundant presence of tissue factor-exposing extracellular vesicles with strong coagulation activity

Extracellular vesicles (EVs) in easily accessible body fluids have emerged as a promising source for liquid biopsy. Although tear collection is fast, safe, and noninvasive, EVs of tear fluid are less studied and their involvement in physiological and pathological processes is largely unknown. The aim of present study was to analyze and characterize EVs in tear fluid at the single-particle level to reveal the population heterogeneity. A laboratory-built nano-flow cytometer (nFCM) was used to analyze the purity, size distribution, and particle concentration of EVs isolated from unstimulated tears (basal tears) upon double ultracentrifugation (17 min at 100,000×g, 4 °C) via side scattering detection. The expression of CD9, CD63, CD81, CD47, CD45, CD24, and EpCAM was assessed via immunofluorescent detection. The EV concentration in tear fluid was measured to be 1.1 ± 0.7 × 10¹¹ particles/mL, which is approximately 100-fold higher than that of plasma EVs. In particular, it was identified for the first time that tears have strong coagulant activity owing to the abundant presence of tissue factor (TF) on tear EVs. The concentration of TF-exposing EVs (4.4 ± 3.1 × 10¹⁰ particles/mL) was found to be approximately 100-fold higher than their counterparts in saliva (4.5 ± 2.1 × 10⁸ particles/mL). We postulate that TF-exposing vesicles in tears might play a role in host defense by promoting clot formation and thus reducing the risk of pathogen invasion. The coagulant activity of tears triggered by TF-exposing EVs could provide a new research perspective for ophthalmic research.

Identifying Cell-Type Specific Genes and Expression Rules Based on Single-Cell Transcriptomic Atlas Data

Single-cell sequencing technologies have emerged to address new and longstanding biological and biomedical questions. Previous studies focused on the analysis of bulk tissue samples composed of millions of cells. However, the genomes within the cells of an individual multicellular organism are not always the same. In this study, we aimed to identify the crucial and characteristically expressed genes that may play functional roles in tissue development and organogenesis, by analyzing a single-cell transcriptomic atlas of mice. We identified the most relevant gene features and decision rules classifying 18 cell categories, providing a list of genes that may perform important functions in the process of tissue development because of their tissue-specific expression patterns. These genes may serve as biomarkers to identify the origin of unknown cell subgroups so as to recognize specific cell stages/states during the dynamic process, and also be applied as potential therapy targets for developmental disorders.

Constitutive and activation-dependent phosphorylation of lymphocyte phosphatase-associated phosphoprotein (LPAP)

Lymphocyte phosphatase-associated phosphoprotein (LPAP) is a small transmembrane protein expressed exclusively in lymphocytes. LPAP is a component of a supramolecular complex composed of the phosphatase CD45, the co-receptor CD4, and the kinase Lck. In contrast to its immunologically important partners, the function of LPAP is unknown. We hypothesized that the biological role of LPAP may be determined by analyzing LPAP phosphorylation. In the present study, we identified LPAP phosphorylation sites by site-directed mutagenesis, phospho-specific antibodies, and protein immunoprecipitation coupled to mass spectrometry analysis. Our results confirmed previous reports that Ser-99, Ser-153, and Ser-163 are phosphorylated, as well as provided evidence for the phosphorylation of Ser-172. Using various SDS-PAGE techniques, we detected and quantified a set of LPAP phosphoforms that were assigned to a combination of particular phosphorylation events. The phosphorylation of LPAP appears to be a tightly regulated process. Our results support the model: following phorbol 12-myristate 13-acetate (PMA) or TCR/CD3 activation of T cells, LPAP is rapidly dephosphorylated at Ser-99 and Ser-172 and slowly phosphorylated at Ser-163. Ser-153 exhibited a high basal level of phosphorylation in both resting and activated cells. Therefore, we suggest that LPAP may function as a co-regulator of T-cell signaling.

Epitope mapping of lymphocyte phosphatase-associated phosphoprotein

Lymphocyte phosphatase-associated phosphoprotein (LPAP) is a transmembrane protein with unknown function. The available data on its close association with phosphatase CD45 and its phosphorylation depending on cell activation suggest that LPAP can play a significant role in the antigenic stimulation of lymphocytes. We have localized three antigenic epitopes of the LPAP molecule that can be detected using monoclonal antibodies prepared earlier. Experiments on reactions of antibodies with point mutants and shortened forms of the LPAP protein revealed regions of the amino acid sequence that correspond to the epitopes recognized by the antibodies.

Lessons learned from gene expression profiling of cutaneous T-cell lymphoma

Gene expression studies of cutaneous T-cell lymphoma (CTCL) span a decade, yet the pathogenesis is poorly understood and diagnosis remains a challenge. This review examines the varied approaches to gene expression analysis of CTCL, with emphasis on cell populations, control selection and expression data collection. Despite discordant results, several dysregulated genes have been identified across multiple studies, including PLS3, KIR3DL2, TWIST1 and STAT4. Here, we provide an overview of the most consistently expressed genes across different studies and bring them together through common pathways biologically relevant to CTCL. Four pathways - evasion of activation-induced cell death, T helper 2 lymphocyte differentiation, transforming growth factor-β receptor expression, and tumour necrosis factor receptor ligands - appear to encompass the most frequently affected genes, hypothetically providing insight into the disease pathogenesis.

A regulatory polymorphism at position -309 in PTPRCAP is associated with susceptibility to diffuse-type gastric cancer and gene expression

PTPRCAP (CD45-AP) is a positive regulator of protein tyrosine phosphatase PTPRC (CD45), which activates Src family kinases implicated in tumorigenesis. Single-nucleotide polymorphism (SNP) rs869736 located at position -309 of the PTPRCAP promoter was associated with susceptibility to diffuse-type gastric cancer in the current case-control study. The minor-allele homozygote was significantly associated with a 2.5-fold increased susceptibility to diffuse-type gastric cancer (P = .0021, n = 252), but not to intestinal-type (P = .30, n = 178), versus the major-allele homozygote, when comparing unrelated Korean patients with healthy controls (n = 406). Nine other SNPs were in nearly perfect linkage disequilibrium (r(2) >or= 0.97) with this SNP, exhibiting the same association, and spread out for 26 kb on chromosome 11q13.1 covering RPS6KB2, PTPRCAP, CORO1B, and GPR152. Among the four genes, however, only PTPRCAP expression was affected by haplotypes of the 10 SNPs. Endogenous transcript levels of PTPRCAP were linearly correlated with copy numbers (0, 1, and 2) of the risk-haplotype (P = .0060) in 12 lymphoblastoid cells derived from blood samples, but those of the other three genes were not. Furthermore, the cancer-risk, minor-allele T of rs869736 increased both promoter activity and specific nuclear protein-binding affinity than the nonrisk, major-allele G in luciferase reporter and electrophoretic mobility shift assays, respectively. Accordingly, the minor allele of rs869736 in the PTPRCAP promoter is associated with increased susceptibility to diffuse-type gastric cancer by increasing PTPRCAP expression, possibly leading to activation of the oncogenic Src family kinases.

Modulation of immune cell signalling by the leukocyte common tyrosine phosphatase, CD45

CD45 is a leukocyte specific transmembrane glycoprotein and a receptor-like protein tyrosine phosphatase (PTP). CD45 can be expressed as several alternatively spliced isoforms that differ in the extracellular domain. The isoforms are regulated in a cell type and activation state-dependent manner, yet their function has remained elusive. The Src family kinase members Lck and Lyn are key substrates for CD45 in T and B lymphocytes, respectively. CD45 lowers the threshold of antigen receptor signalling, which impacts T and B cell activation and development. CD45 also regulates antigen triggered Fc receptor signalling in mast cells and Toll-like receptor (TLR) signalling in dendritic cells, thus broadening the role of CD45 to other recognition receptors involved in adaptive and innate immunity. In addition, CD45 can affect immune cell adhesion and migration and can modulate cytokine production and signalling. Here we review what is known about the substrate specificity and regulation of CD45 and summarise its effect on immune cell signalling pathways, from its established role in T and B antigen receptor signalling to its emerging role regulating innate immune cell recognition and cytokine production.

Expression of CD45 isoforms correlates with differential proliferative responses of peripheral CD4+ and CD8+ T cells

CD4(+) T cells express IL-2 receptor complexes to the same level as CD8(+) T cells when the two T cell populations were stimulated simultaneously. However, the activation of downstream signaling molecules, such as Jaks, was increased in CD8(+) T cells. Although equivalent amounts of CD45, which acts as a Jak phosphatase, was expressed on the two T cell populations, those on the CD8(+) T cells have less protein tyrosine phosphatase activity than those on the CD4(+) T cells. Furthermore, we find that different CD45 isoforms dominate in the two populations; CD45RO on proliferating CD4(+) T cells and CD45RBC on proliferating CD8(+) T cells. In addition, NIH3T3 cells expressing the CD45RBC transgene had more phosphorylated Jak1 and grew faster than those with the CD45RO transgene. Thus, the expression of specific CD45 isoforms on T cells correlates with their proliferative response to IL-2, suggesting that controlling cells expressing specific CD45 isoforms could correct excessive or insufficient immune responses.

CD45-associated protein promotes the response of primary CD4 T cells to low-potency T-cell receptor (TCR) stimulation and facilitates CD45 association with CD3/TCR and lck

Although it is clear that the CD45 tyrosine phosphatase is required for efficient T-cell activation and T-cell development, the factors that regulate CD45 function remain uncertain. Previous data have indicated that there is an association of CD45 with CD4 and the T-cell receptor (TCR) complex controlled by the variable ectodomain of CD45 and, following activation, by high- and low-potency peptides. This suggests that controlling substrate access to CD45 may be an important regulatory mechanism during T-cell activation. In the present study we have examined the role of the transmembrane adapter-like molecule CD45-associated protein (CD45-AP) in regulating the association of CD45 with CD3/TCR and lck, and in regulating primary CD4(+) T-lymphocyte activation. In CD4(+) T cells from CD45-AP-deficient mice, coimmunoprecipitation of CD45 with the CD3/TCR complex, in addition to lck, is significantly reduced compared with wild-type T cells. Functionally, this correlates with a decreased proliferative response, a decrease in interleukin (IL)-2 production, and a decrease in calcium flux upon stimulation with a low-potency altered peptide ligand. However, the response of CD45-AP-deficient T cells to stimulation with a high-avidity agonist peptide was largely intact, except for a modest decrease in IL-2 production. These data suggest that CD45-AP promotes or stabilizes the association of CD45 with substrates and regulates the threshold of T-cell activation.

Expression of CD45 Lacking the Catalytic Protein Tyrosine Phosphatase Domain Modulates Lck Phosphorylation and T Cell Activation

The function of the second protein tyrosine phosphatase domain (D2) in two-domain protein tyrosine phosphatases (PTP) is not well understood. In CD45, D2 can interact with the catalytic domain (D1) and stabilize its activity. Although D2 itself has no detectable catalytic activity, it can bind substrate and may influence the substrate specificity of CD45. To further explore the function of D2 in T cells, a full-length construct of CD45 lacking the D1 catalytic domain (CD45RABC-D2) was expressed in CD45+ and CD45- Jurkat T cells. In CD45- Jurkat T cells, CD45RABC-D2 associated with Lck but, unlike its active counterpart CD45RABC, did not restore the induction of tyrosine phosphorylation or CD69 expression upon T cell receptor (TCR) stimulation. Expression of CD45RABC-D2 in CD45+ Jurkat T cells resulted in its association with Lck, increased the phosphorylation state of Lck, and reduced T cell activation. TCR-induced tyrosine phosphorylation was delayed, and although MAPK phosphorylation and CD69 expression were not significantly affected, the calcium signal and IL2 production were severely reduced. This indicates that the non-catalytic domains of CD45 can interact with Lck in T cells. CD45RABC-D2 acts as a dominant negative resulting in an increase in Lck phosphorylation and a preferential loss of the calcium signaling pathway, but not the MAPK pathway, upon TCR signaling. This finding suggests that, in addition to their established roles in the initiation of TCR signaling, CD45 and Lck may also influence the type of TCR signal generated.

CD45-associated protein inhibits CD45 dimerization and up-regulates its protein tyrosine phosphatase activity

CD45, a receptor-like protein tyrosine phosphatase (PTP), plays an essential role in lymphocyte development and immune responses. Recent evidence suggests that dimerization of CD45 down-regulates its function. However, the mechanisms by which CD45 dimerization is regulated remain unclear, and there is no direct evidence that the PTP activity of CD45 dimers is less than that of monomers. CD45 in lymphocytes associates with CD45-AP (CD45-associated protein). Here we show that T cells from CD45-AP-null mice have a much higher level of CD45 dimers than those of wild-type mice, suggesting that CD45-AP inhibits CD45 dimer formation. This was confirmed with the use of a novel CD45-AP-null T-cell line, ALST-1, that we established from a spontaneous thymic tumor found in a CD45-AP-null mouse. Transfected CD45-AP inhibited CD45 dimer formation in ALST-1 cells in proportion to the amount of CD45-AP expressed. Finally, with the use of microsomal fractions from both mouse thymocytes and ALST-1 transfectants, the PTP activity of CD45 was found to be significantly lower in CD45-AP-negative cells than in CD45-AP-positive cells. Therefore, our results support a model in which binding of CD45-AP to inactive CD45 dimers converts them to active monomers. (Blood. 2004;103:3440-3447)

Apoptosis mediated through CD45 is independent of its phosphatase activity and association with leukocyte phosphatase-associated phosphoprotein

Besides the well-recognized role of CD45 as a major player in TCR signaling, we and others have demonstrated that cross-linking of CD45 with mAbs can induce cell death in T lymphocytes. To investigate the role of CD45 phosphatase activity in apoptosis induction, we expressed either wild-type or phosphatase-dead CD45 molecules in a CD45-deficient BW5147 T cell line. We show here that the phosphatase activity of CD45 was not required for apoptosis triggering after cross-linking of the molecule. It is noteworthy that a revertant of the CD45-negative BW5147 cell line, expressing a truncated form of CD45 lacking most of the cytoplasmic domain, was also susceptible to CD45-mediated death. Moreover, we also demonstrate that leukocyte phosphatase-associated phosphoprotein expression is totally dispensable for CD45-mediated apoptosis to occur. Taken together, these results strongly suggest a role for the extracellular and/or the transmembrane portion of CD45 in apoptosis signaling, which contrasts with the previously reported functions for CD45 in T lymphocytes.

CD45-associated protein is not essential for the regulation of antigen receptor-mediated signal transduction

CD45 is a transmembrane protein tyrosine phosphatase required for signaling through the T-and B-cell antigen receptors. In lymphocytes, CD45 interacts with CD45-associated protein (CD45AP), a 32 000 Mr phosphoprotein, through their respective transmembrane domains. To determine whether CD45AP affects the ability of CD45 to regulate antigen receptor signaling, CD45AP-deficient mice were generated. Thymocyte development was grossly normal. Moreover, the cellularity of the thymus and spleens were normal. CD45 expression on thymocytes and splenocytes, ascertained by flow cytometry, was comparable between CD45AP-deficient mice and littermate controls. In contrast to a previous report (Matsuda et al., J. Exp. Med. 1998 187: 1863 - 1870). CD45AP-deficient and normal thymocytes and splenocytes proliferated similarly in response to various mitogens or antigen receptor cross-linking. Furthermore, thymocyte CD45-associated p56(lck) kinase activity was similar between CD45AP-deficient and normal cells. We conclude that CD45AP is not essential for the regulation of Src-family kinase activity by CD45.

Biochemical and functional analysis of mice deficient in expression of the CD45-associated phosphoprotein LPAP

The role of the CD45-associated phosphoprotein (LPAP / CD45-AP) during an immune response remains unclear. To understand better the function of LPAP we generated LPAP-deficient mice by disrupting exon 2 of the LPAP gene. LPAP-null mice were healthy and did not show gross abnormalities compared to their wild-type littermates. However, immunofluorescence analysis of T and B lymphocytes revealed a reduced expression of CD45, which did not affect a particular subpopulation. In contrast to a recent report (Matsuda et al., J. Exp. Med. 1998. 187: 1863 - 1870) we neither observed significant alterations of the assembly of the CD45 / lck-complex nor of polyclonal T-cell responses. However, lymphnodes from LPAP-null mice showed increased cellularity, which could indicate that expression of LPAP might be required to prevent expansion of lymphocytes in particular lymphatic organs rather than potentiating immune responses.

Interactions of CD45-associated protein with the antigen receptor signaling machinery in T-lymphocytes

CD45 is a transmembrane protein tyrosine phosphatase playing an essential role during T-cell activation. This function relates to the ability of CD45 to regulate p56(lck), a cytoplasmic protein tyrosine kinase necessary for T-cell antigen receptor (TCR) signaling. Previous studies have demonstrated that CD45 is constitutively associated in T-lymphocytes with a transmembrane molecule termed CD45-AP (or lymphocyte phosphatase-associated phosphoprotein). Even though the exact role of this polypeptide is unclear, recent analyses of mice lacking CD45-AP have indicated that its expression is also required for optimal T-cell activation. Herein, we wished to understand better the function of CD45-AP. The results of our studies showed that in T-cells, CD45-AP is part of a multimolecular complex that includes not only CD45, but also TCR, the CD4 and CD8 coreceptors, and p56(lck). The association of CD45-AP with TCR, CD4, and CD8 seemed to occur via the shared ability of these molecules to bind CD45. However, binding of CD45-AP to p56(lck) could take place in the absence of other lymphoid-specific components, suggesting that it can be direct. Structure-function analyses demonstrated that such an interaction was mediated by an acidic segment in the cytoplasmic region of CD45-AP and by the kinase domain of p56(lck). Interestingly, the ability of CD45-AP to interact with Lck in the absence of other lymphoid-specific molecules was proportional to the degree of catalytic activation of p56(lck). Together, these findings suggest that CD45-AP is an adaptor molecule involved in orchestrating interactions among components of the antigen receptor signaling machinery. Moreover, they raise the possibility that one of the functions of CD45-AP is to recognize activated Lck molecules and bring them into the vicinity of CD45.

Interaction between CD45-AP and protein-tyrosine kinases involved in T cell receptor signaling

CD45-AP associates specifically with CD45, a protein-tyrosine phosphatase essential for antigen receptor-mediated signal transduction. CD45 modulates the activity of Src family protein-tyrosine kinases involved at the onset of antigen receptor-mediated signaling by dephosphorylating their regulatory tyrosyl residues. We have shown that lymphocyte responses to antigen receptor stimulation are impaired in CD45-AP-null mice. To examine the possibility that CD45-AP coordinates the interaction between CD45 and its substrates, we investigated the associations of CD45-AP with several protein-tyrosine kinases. Endogenous CD45-AP coimmunoprecipitated with Lck and ZAP-70 in both CD45-positive T cells and their CD45-negative variants after stimulation by antigen receptor ligation. Concomitantly, CD45 coimmunoprecipitated with Lck and ZAP-70 after T cell receptor-mediated stimulation of CD45-positive cells. Recombinant CD45-AP exhibited specific binding to Lck and ZAP-70 protein-tyrosine kinases, but not to Fyn or Csk, in lysates of both CD45-positive and -negative T cells. Specific interactions were demonstrated between the respective recombinant proteins as well. These results demonstrate that CD45-AP associates directly and selectively with Lck and ZAP-70 in response to T cell receptor-mediated stimulation. The associations of CD45-AP with Lck and ZAP-70 may mediate the functional interactions of these kinases with CD45 during antigen receptor stimulation.

Definition of family of coronin-related proteins conserved between humans and mice: close genetic linkage between coronin-2 and CD45-associated protein

Cell adhesion and signal transduction are coordinated processes that may be linked through regulatory elements such as actin-binding proteins. One such protein that may fulfill this role is coronin. In Dictyostelium discoideum, coronin is involved in cellular processes such as mitosis, cell motility, and phagocytosis. In addition, a human coronin, p57, has been described which interacts with the p47 component of phox proteins and may be involved in the formation of phagocytic vacuoles. Here, we describe a family of four mouse proteins which share 38% identity with Dictyostelium coronin and thus are designated coronin-1, -2, -3, and -4. The gene for coronin-2 is localized to mouse chromosome 19, 5' of the gene for CD45-associated protein. All the coronin proteins contain five highly conserved WD domains. However, their carboxyl regions are quite distinct. Three of the four proteins are ubiquitously expressed, whereas coronin-1, the mouse ortholog of p57, demonstrates expression restricted to hematopoietic cells. Comparison of expressed sequence tag cDNAs indicates that coronin-1, -2, -3, and -4 are highly conserved between mice and humans.

Differences in the Immune Response During the Acute Phase of E-55+ Murine Leukemia Virus Infection in Progressor BALB and Long Term Nonprogressor C57BL Mice

E-55+ murine leukemia virus infection of both progressor (BALB) and long term nonprogressor (C57BL) mouse strains is characterized by an acute and a persistent phase of infection. During the acute phase, progressor strains require CD8+ T cells to decrease virus burden, whereas the long term nonprogressor strains do not. In the present studies the immune response in BALB and C57BL mice during the acute phase of E-55+ murine leukemia virus infection was examined. The results demonstrate that BALB mice produce both IL-4 and IFN-γ, in contrast to C57BL mice, which produce only IFN-γ. In BALB mice, IL-4 production results in the absolute requirement for CD8+ T cells to reduce the virus burden during the acute phase of infection. The anti-virus immune response in these mice is IFN-γ dependent. On the other hand, C57BL mice do not produce IL-4 and, in the absence of both CD8+ T cells and IFN-γ, still generate an effective anti-virus immune response. Genetic studies suggest that these distinct immune responses are regulated by more than one non-MHC-linked gene. Two candidate regions that may encode this gene(s), located on chromosomes 7 and 19, respectively, were identified by recombinant inbred strain linkage analysis.

Chimeric CD4/CD44 molecules associate with CD44 via the transmembrane region and reduce hyaluronan binding in T cell lines

Cells of the immune system tightly regulate the binding ability of cell adhesion molecules. The binding of the extracellular matrix component hyaluronan to CD44 is no exception, yet the mechanisms that regulate its binding are poorly understood. In this study a chimeric CD4/CD44 molecule, containing the extracellular domain of CD4 and the transmembrane and cytoplasmic domains of CD44, was expressed in two CD44+ mouse T lymphoma cell lines, BW5147 and T28. This resulted in the reduced ability of endogenous CD44 to constitutively bind hyaluronan. Immunoprecipitation of the chimeric protein in 1 % Brij-96 indicated an association between the chimera and endogenous CD44. Using various chimeric CD4/CD44 molecules, the transmembrane region of CD44 was found to mediate this association. In addition, the association of chimeric CD4/CD44 molecules with endogenous CD44 correlated with reduced hyaluronan binding. Thus, the transmembrane region of CD44 is required for the association with CD44 molecules in the cell membrane and we propose that the self-association of CD44 molecules occurs on the T cell surface to promote hyaluronan binding. Cellular events altering the interactions of the transmembrane region of CD44 thus have the potential to regulate the hyaluronan binding ability of CD44.

Disruption of lymphocyte function and signaling in CD45-associated protein-null mice

CD45-AP specifically associates with CD45, a protein tyrosine phosphatase essential for lymphocyte differentiation and antigen receptor-mediated signal transduction. CD45 is thought to mediate antigen receptor signaling by dephosphorylating regulatory tyrosine residues on Src family protein tyrosine kinases such as Lck. However, the mechanism for regulating CD45 protein tyrosine phosphatase activity remains unclear. CD45-AP-null mice were created to examine the role of CD45-AP in CD45-mediated signal transduction. T and B lymphocytes showed reduced proliferation in response to antigen receptor stimulation. Both mixed leukocyte reaction and cytotoxic T lymphocyte functions of T cells were also markedly decreased in CD45-AP-null mice. Interestingly, the interaction between CD45 and Lck was significantly reduced in CD45-AP-null T cells, indicating that CD45-AP directly or indirectly mediates the interaction of CD45 with Lck. Our data indicate that CD45-AP is required for normal antigen receptor signaling and function in lymphocytes.

The role of CD45 and CD45-associated molecules in T cell activation

CD45 (lymphocyte common antigen) is a receptor-linked protein tyrosine phosphatase that is expressed on all leucocytes, and which plays a crucial role in the function of these cells. On T cells the extracellular domain of CD45 is expressed in several different isoforms, and the particular isoform(s) expressed depends on the particular subpopulation of cell, their state of maturation, and whether or not they have previously been exposed to antigen. It has been established that the expression of CD45 is essential for the activation of T cells via the TCR, and that different CD45 isoforms display a different ability to support T cell activation. Although the tyrosine phosphatase activity of the intracellular region of CD45 has been shown to be crucial for supporting signal transduction from the TCR, the nature of the ligands for the different isoforms of CD45 have been elusive. Moreover, the precise mechanism by which potential ligands may regulate CD45 function is unclear. Interestingly, in T cells CD45 has been shown to associate with numerous molecules, both membrane associated and intracellular; these include components of the TCR-CD3 complex and CD4/CD8. In addition, CD45 is reported to associate with several intracellular protein tyrosine kinases including p56lck and p59fyn of the src family, and ZAP-70 of the Syk family, and with numerous proteins of 29-34 kDa. These CD45-associated molecules may play an important role in regulating CD45 tyrosine phosphatase activity and function. However, although the role of some of the CD45-associated molecules (e.g. CD45-AP and LPAP) has become better understood in recent years, the role of others still remains obscure. This review aims to summarize recent findings on the role of CD45 and CD45-associated molecules in T cell activation, and to highlight issues that seem relevant to ongoing research in this area.

Definition of amino acids sufficient for plasma membrane association of CD45 and CD45-associated protein

The transmembrane protein tyrosine phosphatase CD45 functions to activate Src-family member kinase activity in T lymphocytes. The inability to activate p56(lck) in CD45-deficient cells results in a higher threshold of signaling through the T cell receptor. The lymphoid-specific CD45-associated protein, CD45AP, interacts with CD45 through transmembrane interactions. Cells lines and mice deficient in CD45 express CD45AP mRNA, yet the protein is poorly expressed, indicating that CD45 is required for efficient expression of CD45AP. Pulse-chase analysis indicates that CD45 associates with CD45AP within minutes of biosynthesis. Cell surface labeling and coimmunoprecipitation demonstrate that CD45AP associates with surface-expressed CD45. Therefore, CD45AP is localized to the plasma membrane. To further characterize this interaction, chimeric proteins containing mutations in CD45 transmembrane regions were expressed, and their ability to associate with CD45AP was determined. Alanine-scan mutations of the CD45 transmembrane region demonstrate that no single amino acid is essential for the interaction with CD45AP. However, the expression of chimeric transmembrane regions indicates that a minimum of three and a maximum of eight amino acids in this region are sufficient to allow interaction with CD45AP.

Role of phosphatases in lymphocyte activation

Many lymphocyte signaling pathways are regulated by protein tyrosyl phosphorylation, which is controlled by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Substantial progress has been made in defining the functions of lymphocyte PTPs. Individual PTPs can enhance or diminish cell signaling levels. The transmembrane PTP CD45 is a key positive element in multiple lymphocyte signaling pathways in vivo. New insights into the function of individual CD45 isoforms have emerged. Anti-CD45 antibodies with potent immunosuppressant activity have been identified, suggesting that CD45 may be a propitious target for drug design. Progress has also been made in elucidating the function and targets of specific nontransmembrane PTPs, particularly those with Src homology 2 domains.

Reconstitution of interactions between protein-tyrosine phosphatase CD45 and tyrosine-protein kinase p56(lck) in nonlymphoid cells

To further understand the functional interactions between CD45 and p56(lck) in T-cells, we stably reconstituted their expression in a nonlymphoid system. The results of our analyses demonstrated that CD45 could dephosphorylate tyrosine 505 of p56(lck) in NIH 3T3 fibroblasts. As is the case for T-cells, removal of the unique domain of p56(lck) interfered with dephosphorylation of tyrosine 505 in fibroblasts, further stressing the importance of this region in the interactions between CD45 and p56(lck). The ability of CD45 to dephosphorylate tyrosine 505 in NIH 3T3 cells was also greatly influenced by the catalytic activity of p56(lck). Indeed, whereas CD45 provoked dephosphorylation of kinase-defective Lck molecules in this system, it failed to stably dephosphorylate kinase-active p56(lck) polypeptides. Finally, our studies showed that CD45 was also able to inhibit the oncogenic potential of a constitutively activated version of p56(lck) in NIH 3T3 cells. This effect did not require the Lck unique domain and apparently resulted from selective dephosphorylation of substrates of activated p56(lck) in fibroblasts. In addition to providing insights into the nature and regulation of the interactions between CD45 and p56(lck) in T-cells, these results indicated that CD45 clearly has the capacity to both positively and negatively regulate p56(lck)-mediated functions in vivo.

Regulation of cell signaling by the protein tyrosine phosphatases, CD45 and SHP-1

An equilibrium between positive and negative regulation of immunoreceptor signaling leads to the proper execution of lymphocyte activation. Tyrosine phosphorylation is the initial event in antigen receptor-induced lymphocyte activation. It is generally accepted that protein tyrosine kinases are involved in positive regulation, whereas protein tyrosine phosphatases are important for the negative regulation of tyrosine phosphorylation-dependent processes. However, the interaction between protein tyrosine kinases and protein tyrosine phosphatases is complex. This article discusses the role of two protein tyrosine phosphatases. CD45 and SHP-1, in the regulation of immunoreceptor signaling. SHP-1 acts as a negative regulator for several immunoreceptors, including those for T- and B-cell antigen receptors. The major role of CD45 is in the positive regulation of T- and B-cell antigen receptor signaling.

CD45-associated protein is a lymphocyte-specific membrane protein expressed in two distinct forms

CD45-AP is a recently identified CD45-associated protein. The two proteins interact specifically through their respective transmembrane segments. Northern hybridization analysis of CD45+ T lymphocytes and their CD45- variants demonstrated that the production of CD45-AP and CD45 mRNA is regulated independently. On the other hand, Western blotting analysis indicated that the CD45-AP protein has a shorter half-life in the absence of CD45 in three out of four variants. Similar analysis of various types of leukocytes demonstrated that CD45-AP is expressed in T, B, and pre-B cells, but not in plasma cells or cells of the monocyte/macrophage lineage. Two forms of CD45-AP mRNA exist in all types of CD45-AP-expressing lymphocytes analyzed. One form corresponds to the previously reported CD45-AP cDNA and the other form encodes an additional 12 amino acids at the N terminus. The two CD45-AP proteins are identical in their capacity for specific binding to CD45, but employ different mechanisms for endoplasmic reticulum membrane translocation.

Demonstration of a direct interaction between p56lck and the cytoplasmic domain of CD45 in vitro

p56lck is a potential in vivo substrate for the tyrosine-specific phosphatase, CD45. In this study, recombinant purified p56lck was found to specifically associate with recombinant CD45 cytoplasmic domain protein, but not to the cytoplasmic domain of another related tyrosine phosphatase, receptor protein-tyrosine phosphatase alpha. Under equilibrium binding conditions, the binding was saturable and occurred at a 1:1 molar stoichiometry. A fusion protein containing only the amino-terminal region of p56lck (residues 34-150) also bound to recombinant CD45, and further analysis of this region indicated that glutathione S-transferase fusion proteins of the unique amino-terminal region and the SH2 domain, but not the SH3 domain of p56lck, bound to recombinant CD45. The SH2 domain protein bound with a higher affinity than the amino-terminal region, but both were able to compete for the binding of p56lck to CD45, and when added together worked synergistically to compete for p56lck binding. The SH2 domain interaction with CD45 was specific as glutathione S-transferase-SH2 fusion proteins from p85 alpha subunit of phosphatidylinositol 3-kinase and SHC did not bind to CD45. In addition, this interaction occurred in the absence of any detectable tyrosine phosphorylation on CD45, suggesting a nonconventional SH2 domain interaction.